1. Field of the Invention
The present invention relates to a carbon steel wire for steel wire cords, wire ropes, etc., a steel product as a raw material for said steel wire, and a process for producing them. This carbon steel wire is finished without heat treatment (such as bluing) after cold rolling.
2. Description of the Related Art
Automotive steel radial tires are reinforced with steel wires such as cord wires and bead wires. Their constituent is a strand composed of twisted high-carbon steel wires, each measuring 0.2 mm in diameter and having a strength greater than 310 kgf/mm.sup.2.
Each wire constituting the strand is produced by wire drawing from a eutectoid or hyper-eutectoid high-carbon steel. The step of wire drawing is followed by patenting, pickling, brass plating (to ensure good adhesion to rubber). Final drawing gives a thin wire about 0.2 mm in diameter. The patenting treatment transforms the austenitic structure into the fine pearlite structure at about 500-550.degree. C., thereby making the steel tougher. Automotive tires recently call for improved durability and said steel wires also call for higher strength than before. An effective way to increase strength is to increase the carbon content. However, a mere increase in carbon content results in longitudinal cracking when the wire is twisted. Longitudinal cracking can be effectively prevented by incorporation with chromium. Japanese Patent Laid-open No. 194147/1990 discloses incorporation with 0.10-0.30% of chromium. Japanese Patent Laid-open No. 049592/1994 discloses incorporation with chromium in a specific amount determined by the boron content. The latter is intended to promote the growth of cementite in pearlite and to improve ductility and fatigue characteristics.